The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of CT imaging using multi-spot emission sources.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
It is generally desirable to have increased speed, coverage, and resolution of CT scanners. In recent years, manufacturers have improved scanners by increasing the gantry speed, by reducing the pixel size, and by extending the coverage of the detectors in the Z direction by extending the length of the detector array in the Z direction. This approach has resulted in development of CT systems that have larger detector arrays. Larger detector arrays, however, may be undesirable for a number of reasons. For instance, large detector arrays add cost and complexity to a CT system, not only in the detector components themselves, but in the data acquisition systems required to read out the increased number of channels. The increased detector array size also results in an increased mass of the overall detector, thereby resulting in increased mechanical stresses in the components of the CT system.
A complete dataset is typically acquired during a rotation of a CT gantry through approximately 180 degrees, thereby defining the temporal resolution of a CT scanner, ignoring cone angles. Accordingly, the temporal resolution may be improved by spinning the gantry faster. However, mechanical stresses therein substantially increase with increased gantry speed, thereby imposing practical limits on the upper speed of the gantry.
As detector arrays get longer in the Z direction, an increase in the cone angle occurs as well. The cone angle is the angle, along the Z direction, between the focal spot and the edges of the detector array. The increase in cone beam angle leads to cone beam artifacts in reconstructed images. Beyond a certain limit, the cone beam becomes severe, and increased scan coverage may not be accomplished by simply increasing the length of the detector array along the Z direction.
It is generally desired, as well, to obtain scan data exclusively from a cardiac region of a patient, as well from a larger patient field-of-view, while reducing the x-ray dose that a patient is exposed to during a CT scan. Traditional single spot CT scanners typically use a bowtie filter to make the detected flux somewhat uniform throughout the detector array. The bowtie filter results in scattered radiation that is not useful for the purpose of image acquisition. Because the amount of scatter radiation tends to be high for single spot CT sources, the detectors have a collimator positioned to attenuate, or block, x-rays that do not derive from the primary source. The collimator, as well, results in a loss of dose efficiency that, for given image quality, results in increased dose to the patient.
Therefore, it would be desirable to design a CT apparatus and method to improve image quality while increasing Z coverage of a subject and decreasing dose to the subject.